Membrane contactors allow two different phases, such as a gaseous phase and a liquid phase, to be in direct contact with each other without involving additional reactions that could change the properties of both phases. A membrane contactor differs from conventional membranes that are used for liquid filtration processes, such as reverse osmosis, ultrafiltration, and microfiltration. These membranes are not capable of removing gases from liquid phases, and they are not capable of trans­ferring gases into liquid phases. The process of dispersing gases into liquid phase involves three stages, i. e., (1) The transfer of gases from the gas phase to the gas-membrane interphase; (2) transfer of the gases through the membrane to the liquid-membrane interface; and (3) transfer of the gases into the liquid bulk phase. This can be illustrated as shown in Fig. 14.1.

Unlike a conventional membrane, the main driving force for a membrane contac­tor is a concentration gradient rather than a pressure gradient. Membrane contactors were researched actively a few years ago for use in the advanced separation and

purification processes in gas/liquid and liquid/liquid industries to reduce the depen­dency on conventional separation devices. Conventional separation processes, such as absorption towers and column mixers, achieve low mass transfer efficiency due to the low interfacial contact area between the two different phases and the low dispersion rate (when a device is used to aid the dispersion process) due to various side effects, including flooding, the formation of emulsions, and foaming, all of which can be solved by using a membrane contactor. However, membrane contac­tors also have their limitations. In the application of a membrane contactor to aid the mitigation of CO2 by microalgae, the membrane device must be hydrophobic to prevent the microalgae cells from clogging the pores of the membrane.